# Coordinate system

## Transcription

Coordinate system

Coordinate system Sudarto Laboratorium Pedologi dan Sistem Informasi Sumberdaya Lahan Jurusan Tanah, Fakultas Pertanian Universitas Brawijaya Earth’s surface is three-dimensional • Feature location expressed in latitude and longitude Maps are two-dimensional • Map feature location identified with x- and y- coordinates How do we register the three-dimensional Earth to the two-dimensional map? • Location reference system for spatial features on earth’s surface • Angular measurement from an origin to a given point • Latitude (parallels) • Longitude (meridians) Parallels of Latitude Meridians of Longitude Graticular Network b: Latitude a: longitude A mathematical model of the earth must be selected. Ellipsoid / Spheroid The mathematical model must be related to real-world features. Datum Real-world features must be projected with minimum distortion from a round earth to a flat map; and given a grid system of coordinates. Map Projection A mathematical model of the earth must be selected Simplistic - A round ball having a radius big enough to approximate the size of the earth. Reality - Spinning planets swell at the equator with correlative flattening at the poles. e.g. Shape of Earth not perfectly circular Ellipsoid Major axis (a) Minor axis (b) Flattening (f) (a-b)/a If a=b, the ellipsoid is a spheroid A mathematical approximation to the shape of the Earth Semi minor axis Semi mayor axis The Geoid is an equipotential surface of the Earth gravity field that most closely approximates the mean sea level. The ellipsoid is an approximation to the Geoid. Using different approximations will result in different accuracies. Coordinate systems are applied to the simpler model of a ellipsoid / spheroid. The problem is that actual measurements of location conform to the geoid surface and have to be mathematically recalculated to positions on the ellipsoid / spheroid. A mathematical model must be related to real-world features A smooth mathematical surface that fits closely to the mean sea level surface throughout the area of interest. The surface to which the ground control measurements are referred. •Provides a frame of reference for measuring locations on the surface of the earth. A Datum: is a model of the earth and describes the relationship (origin & orientation) between the coordinate system and the Earth. A Datum is defined by: The size & shape of the model (Ellipsoid) The position & orientation of the model (Ellipsoid) in relation to the physical surface of the Earth. To determine latitude and longitude, surveyors level their measurements down to a surface called a geoid. The geoid is the shape that the earth would have if all its topography were removed. Or more accurately, the shape the earth would have if every point on the earth's surface had the value of mean sea level. The earth's spheroid deviates slightly for different regions of the earth. Using a wrong datum can result in considerable position errors. The definition of a datum can be based on local or global considerations Different nations and agencies may use different Datums • GRS80 (North America) • Clark 1866 (North America • WGS84 (GPS World-wide) • International 1924 (Europe) • Bessel 1841 (Europe) The datum on which GPS coordinates are based and probably the most common datum for GIS data sets with global extent. Geodetic Latitude & Longitude on an ellipsoid (Lat φ, Long λ, Ellipsoid height h) Horizontal datums are the reference values for a system of location measurements. Vertical datums are the reference values for a system of elevation measurements. Chosen Ellipsoid Geodetic Coordinate System (Lat and Long) The Height is defined as the distance from a chosen reference system along the perpendicular to a surface: Geoid & ellipsoid Ellipsoid (Ellipsoid height h): • a mathematical reference • Provides ellipsoidal heights The Geoid (Orthometric height H): • a physical reference • Provides Orthometric Heights (elevation) GPS provides ellipsoidal heights H: orthometric height (elevation) h: ellipsoidal height N: geoid height H=h-N The job of a vertical datum is to define where zero elevation is, this is usually done by determining mean sea level. Chosen Geoid Chosen Ellipsoid Geoide Height (N) Ellipsoid height (h) Orthometric Height (H: Elevation) Not uniform: – Distances and measures are not accurate Distance of 60° long at 40° latitude 1° longitude: – Equator= 111 km – 60° lat. = 55.8 km – 90° lat. = 0km Distance of 60° long at equator Projecting a Sphere Onto a Plane Curved Earth Geographic coordinates: f, l (Latitude & Longitude) Flat Map Cartesian coordinates: x,y (Easting & Northing) Real-world features must be projected with minimum distortion from a round earth to a flat map; and given a grid system of coordinates. Y X (Xo,Yo) (ooλo) A map projection transforms latitude and longitude locations to x,y coordinates. Map projection is the process of transforming a 3D world into 2D map. Map projection is based on Ellipsoid We often use Spheroid model to illustrate the concept Azimuthal projection Conical projection Cylindrical projection tangent secant Different Projections Preserve Different Properties of the Earth • • • • Directions Distances Shapes Areas If some properties are maintained, errors in others may be exaggerated Mercator Projection Transvrese Mercator Projection Divide the world into sixty vertical strips, each spanning 6 degrees of longitude. Apply a custom Transverse Mercator projection to each strip and use false eastings and northings to make all projected coordinates positive. Universal Transverse Mercator (UTM) Setia tempat di muka bumi terletak pada zone tertentu Sampai ketemu minggu depan Selesaikan tugas minggu yang lalu Lengkapi atribut database dengan: Koordinat (UTM) Y dan Y nya (bisa dilihat pada image Google Earth yang sudah terkoreksi) Atribut lain yang terkait